AN557 datasheet(36/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

36/52

It is necessary much attention also into the choice input capacitors. Also them be at low ESR, because

they must sustain high current ripples. Such current ripples in presents of an inadeguate ESR, would pro-

duce a heating of the capacitor itself (which could affect on the reliability of the component, since in gen-

eral it is sensitive to temperature.

Therefore choosing input capacitor at low ESR is necessary for problems of reliability. In fact such capac-

itors, when used in applications that make use a mains transformer, must support quite elevated peak cur-

rent for short periods a double the mains frequency and the same time be capable to deliver the

instantaneous peak of energy to the load at the switching frequency.

Some other considerations of a general nature can be done on low ESR capacitors. For example of equal

value and type (i.e.: 220

µF - EKR), the ESR of the capacitor decreases at the increasing of its value volt-

age rating, just like its RMS current.

Still, two capacitors of the same value, connected in parallel, withstand an RMS current higher then the

only one of double value, and with the same voltage rating.

When however, more capacitors are connected together in parallel, it is important to design with care the

layout of the printed circuit, in order to distribute as eventy as possible amongst between the different ca-

pacitors the total current ripple.

This is used to avoid dangerous current unbalances in the distribution of the total current between the var-

ious capacitors charging some more others, that could damage the reliability of the system.

Often it is very difficult to know exactly the RMS current flowing throught the capacitors. To know if the

operating condition is a "safe"operating condition or not, a measurement of the package temperature of

the capacitor should be done.

The following table 3 and 4, included in the dat-abook of electrical Roederstain, shows the maximum RMS

current sustainable by the EKR and EKE capacitor versus the ambient temeperature and overtemperature

allowed on the capacitor package.

Table 3. Low-voltage electrolytic capacitors for switch-mode power supplies with low impedance

values, radial, polarized styles

Rated cap.

(

µF)

Rated volt.

(V DC)

Dimensions

D x L (mm)

(nominal

dimen-

sions)

Dissipation

factor tan

(100Hz;

20°C) Lim.

Values

Impedance

Z (

Ω)

(10KHz;

20°C)

(Lim. val-

ues)

Impedance

Z (

Ω)

(10KHz;

20°C)

(Lim. val-

ues)

Admissible

ripple curr.

(mA/100Hz)

85°C

Admissible

ripple curr.

(mA/eff/10-

100Hz)

85°C

EKR

100

220

470

8.7 x 12.7

10 x 12.7

10 x 20

0.12

0.85

0.39

0.20

0.65

0.31

0.18

160

300

530

250

450

800

100

220

470

10 x 12.7

10 x 16

12.5 x 20

0.11

0.60

0.32

0.16

0.40

0.25

0.13

200

350

600

300

550

900

100

220

470

10 x 12.7

10 x 16

12.5 x 20

0.09

0.5

0.25

0.13

0.35

0.17

0.09

250

450

650

400

700

1000

100

220

470

10 x 16

12.5 x 20

12.5 x 30

0.08

0.4

0.17

0.09

0.23

0.13

0.08

450

650

1000

700

1000

1500

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